Method and apparatus for implementing a power converter input terminal voltage discharge circuit

ABSTRACT

A power converter includes an energy transfer element and a circuit coupled to the input of the power converter. The circuit includes a switch and a control circuit capable of detecting whether an AC voltage source is coupled to the input, or uncoupled from the input within a first predetermined time. The control circuit is coupled to drive the switch in a first operating mode when the AC voltage source is coupled, drive the switch in a second operating mode when the AC voltage source is uncoupled, and is capable of discharging a capacitance coupled between input terminals of the power converter to a threshold voltage level through a discharge path and through the switch within a second predetermined time. An RC time constant of the discharge path is less than or equal to one second and the threshold voltage level is less than or equal to ten volts.

REFERENCE TO PRIOR APPLICATION(S)

This is a continuation of U.S. application Ser. No. 14/717,620, filedMay 20, 2015, now pending, which is a continuation of U.S. applicationSer. No. 14/100,882, filed Dec. 9, 2013, now U.S. Pat. No. 9,065,340,which is a continuation of U.S. application Ser. No. 13/345,257, filedJan. 6, 2012, now U.S. Pat. No. 8,624,562, which is a continuation ofU.S. application Ser. No. 12/533,977, filed Jul. 31, 2009, now U.S. Pat.No. 8,115,457. U.S. application Ser. No. 14/717,620 and U.S. Pat. Nos.8,115,457, 8,624,562, and 9,065,340 are hereby incorporated byreference.

BACKGROUND INFORMATION Field of the Disclosure

The present invention relates generally to circuits that dischargecapacitance present between input terminals of a power system when anelectrical energy source is disconnected from the power system inputterminals. More specifically, the present invention relates to circuitsthat discharge EMI filter capacitors coupled across the input of powersystems when a source of ac voltage is disconnected from the powersystem input terminals.

Background

Power systems may be used for a multitude of purposes and applications.Example power systems include power converters where input and outputpower is electrical such as power supplies. Other example power systemsinclude power converters where input power is electrical and outputpower is primarily mechanical such as motor control systems. Powerconverters are typically coupled to a source of electrical energy thatapplies a voltage across the input terminals of the power converter.This source of electrical energy can be a dc or ac source. One class ofpower converters are switched mode power converters.

Switched mode power converters generate electromagnetic interference(EMI) during their operation. Switched mode power converters thereforeinclude EMI filters that are designed to reduce, to acceptable levels,the amount of EMI that is coupled to the source of electrical energy.Many EMI filters include capacitors that are coupled across the inputterminals of the power converter. In cases where the electrical energysource is an ac source, these capacitors may be safety rated capacitorssuch as X capacitors that are rated to allow direct connection across anac electrical energy source prior to any input fuse of the powerconverter. The rugged nature of these X capacitors allows to them beingused directly across the ac line regardless of the position of the inputfuse of the power converter.

When the source of electrical energy is disconnected, the X capacitorcan stay charged at a high voltage. If the X capacitor is of a largeenough value, the energy stored on this capacitor can pose a safety riskto anyone that touches the input terminals of the power converter afterthe source of electrical energy has been disconnected. Internationalsafety standards typically stipulate therefore that if the total EMIfilter capacitance is above a threshold value (typically 0.1 uF), thenthe voltage across the input terminals of the power supply must bereduced to a safe value within a specific period of time. Typically thisrequirement is achieved by permanently connecting one or more resistorsacross the X capacitor terminals. International safety standards, suchas for example EN60950-1, stipulate that the time constant of the Xcapacitor capacitance and the total resistance coupled across the Xcapacitor is less than or equal to 1 second.

Existing resistive discharge circuits of the type described above arelow cost and rugged but dissipate power continually in the resistancewhen the source of electrical energy is connected to the input terminalsof the power system.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a schematic illustrating generally an example power converteremploying a discharge circuit to discharge a capacitance between powerconverter input terminals when a source of electrical energy isdisconnected from the power converter input terminals in accordance withthe teachings of the present invention.

FIG. 2 shows one example of a block diagram of a discharge circuit todischarge a capacitance between a power converter input terminals when asource of electrical energy is disconnected from the power converterinput terminals in accordance with the teachings of the presentinvention.

FIG. 3 shows waveforms associated with an example discharge circuit todischarge a capacitance between a power converter input terminals when asource of electrical energy is disconnected from the power converterinput terminals in accordance with the teachings of the presentinvention.

FIG. 4 shows further waveforms associated with an example dischargecircuit to discharge a capacitance between a power converter inputterminals when a source of electrical energy is disconnected from thepower converter input terminals in accordance with the teachings of thepresent invention.

FIG. 5 is a schematic illustrating an example of a discharge circuit todischarge a capacitance between a power converter input terminals when asource of electrical energy is disconnected from the power converterinput terminals in accordance with the teachings of the presentinvention.

FIG. 6 is a flowchart illustrating an example method for discharging acapacitance between a power converter input terminals when a source ofelectrical energy is disconnected from the power converter inputterminals in accordance with the teachings of the present invention.

FIG. 7 is an example circuit schematic of a power converter circuitemploying a control circuit coupled to drive a switch in a firstoperating mode when a source of electrical energy is coupled to thepower converter input terminals and a second operating mode when thesource of electrical energy is uncoupled from the power converter inputterminals in accordance with the teachings of the present invention.

FIG. 8 is another example circuit schematic of a power converter circuitemploying a control circuit coupled to drive a switch in a firstoperating mode when a source of electrical energy is coupled to thepower converter input terminals and a second operating mode when thesource of electrical energy is uncoupled from the power converter inputterminals in accordance with the teachings of the present invention.

FIG. 9 is a flowchart illustrating an example method for controlling acontroller and a switch used in a power converter such that energy istransferred from an input to an output of the power converter during afirst operating condition and where the switch conducts current withouttransferring energy from an input to an output during a second operatingcondition in accordance with the teachings of the present invention.

FIG. 10 is a schematic illustrating an example power converter employingdischarge circuit, integrated with a power converter control circuit, todischarge a capacitance between a power converter input terminals when asource of electrical energy is disconnected from the power converterinput terminals in accordance with the teachings of the presentinvention.

FIG. 11 is a schematic of an example discharge circuit to discharge acapacitance between power converter input terminals when a source ofelectrical energy is disconnected from the power converter inputterminals in accordance with the teachings of the present invention.

FIG. 12 is a schematic illustrating an example motor control powerconverter system employing a discharge circuit to discharge acapacitance between a power converter input terminals when a source ofelectrical energy is disconnected from the power converter inputterminals in accordance with the teachings of the present invention.

FIG. 13 shows a schematic diagram of another example discharge circuitto discharge a capacitance between power converter input terminals whena source of electrical energy is disconnected from the power converterinput terminals in accordance with the teachings of the presentinvention.

DETAILED DESCRIPTION

Methods and apparatuses for implementing a discharge circuit fordischarging a capacitance existing between input terminals of a powersystem when a source of electrical energy is uncoupled from the inputterminals of the power system are described. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparent,however, to one having ordinary skill in the art that the specificdetail need not be employed to practice the present invention. In otherinstances, well-known materials or methods have not been described indetail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. Particular features, structures or characteristics may beincluded in an integrated circuit, an electronic circuit, acombinational logic circuit, or other suitable components that providethe described functionality. In addition, it is appreciated that thefigures provided herewith are for explanation purposes to personsordinarily skilled in the art and that the drawings are not necessarilydrawn to scale.

The typical technique used to discharge the EMI filter capacitors (oftenX class safety capacitors) that appear across the input terminals ofmany power systems such as switched mode power supplies or switchingmotor control systems, is to place resistors coupled across the inputterminals of the power system. These resistors are coupled in a positionto provide a discharge current path for any energy left stored in theEMI filter capacitors after an electrical energy or electrical powersource is disconnected from the power converter input terminals. In oneexample the electrical energy source is a mains ac voltage source havingan rms voltage level in the 85 to 264 Vac range.

However emerging energy efficiency standards have created a need for asolution that substantially eliminates the power dissipation in thesedischarge resistors while the electrical energy source is stillconnected to the power system input terminals. Examples in accordancewith the teachings of the present invention provide such a solutionvirtually eliminating dissipation in the discharge resistors whileallowing the EMI filter capacitors to be discharged as required when thesource of electrical energy is disconnected from the input terminals ofthe power system.

Various embodiments in accordance with the teachings of the presentinvention are described primarily using switched mode power convertersand motor control power systems as examples. However it is appreciatedthat in general all the teachings in accordance with the teachings ofthe present invention discussed below can be applied to any systemcoupled to a source of electrical energy where the capacitance coupledbetween input terminals of the circuit poses a risk of electrical shockif left charged when the source of electrical energy is uncoupled fromthe input to the system.

To illustrate, FIG. 1 shows generally a schematic of an example powerconverter 100 in accordance with the teachings of the present invention.A source of electrical energy 160 provides an input voltage 121 and iscoupled to input terminals 120 and 140. As shown, power converter 100includes a discharge circuit 104 that will be discussed in more detailbelow. Power converter 100 also includes two power conversion stages inthe illustrated example. The first is power factor correction (PFC)stage 111 and the second is dc-dc conversion stage 112. In the example,dc-dc stage 112 includes a main output 118 and a standby output 119,which is typical of many power converters in, for example, personalcomputers, televisions and the like. In one example, integrated circuitpackage 114 is a multi-die integrated circuit package that includescontroller 113 and switches 115, 116 and 117, which are coupled to themain output 118 and standby output as shown. In another example, it isappreciated that controller 113 and switches 115, 116 and 117 may beincluded in a single monolithic integrated circuit. Controller 113drives switches 115 and 116 to regulate energy flow to main output 118and controller 113 drives switch 117 to regulate energy flow to standbyoutput 119. Similarly in the example, controller 109 and switch 110 areincluded in an integrated circuit package 108. Controller 109 drivesswitch 110 to regulate a flow of energy to the output of PFC conversionstage 111 that provides the input to dc-dc conversion stage 112.

In the example of FIG. 1, the discharge circuit 104 is coupled acrosscapacitor 102, which in one example includes one or more X capacitors.In the example, the power converter input fuse 105 is coupled betweenthe capacitor 102 and the other EMI filter components 106, which forexample could include one or more common mode filter chokes, inductors,Y capacitors and even additional X capacitors. In the example, thedischarge circuit 104 includes a control circuit 128 and a switch 130.In one example, control circuit 128 and switch 130 is included in anintegrated circuit. In one example, switch 130 is an ac switch thatincludes two re-channel MOSFETs 122 and 123 as shown. It is appreciatedthat in other embodiments, other ac switches constructed of for examplebipolar transistors, thyristors, Triodes for alternating current(triacs), Diodes for alternating currents (diacs) or p channel MOSFETscould be employed whilst still benefiting from the teachings of thepresent invention.

In the example, drain terminal 125 of a first MOSFET 122 is coupledthrough resistor 101 to a first input terminal 120 of the powerconverter 100. Drain terminal 126 of a second MOSFET 123 is coupledthrough resistor 103 to a second input terminal 140 of the powerconverter 100 with the source terminals of MOSFETs 122 and 123 coupledtogether. In one example, controller 128 senses that the electricalenergy source 160 is connected to power converter 100 input terminals120 and 140 through, for example, connections 141 and 142. In oneexample, control circuit 128 senses that the voltage between connections141 and 142 reverses within a maximum time period to determine that theelectrical energy source 160 is still connected to the input of powerconverter 100. In one example, the maximum time period is approximately20 milliseconds.

In the illustrated example, the control circuit 128 drives the switch130 to have a high average impedance when the electrical energy source160 is coupled across the input terminals 120 and 140. If the voltagebetween connections 141 and 142 does not reverse within a maximum timeperiod, it is assumed that the electrical energy source 160 is no longerconnected to the input of power converter 100. Under this condition, inone example, the control circuit 128 is coupled to drive the switch 130such that capacitance 102 is discharged to below a threshold voltage inless than a maximum period of time. In one example, the thresholdvoltage is a safety extra low voltage (SELV) level. In one example, whencontroller 130 detects that the electrical energy source 160 isdisconnected from input terminals 120 and 140, the controller 128 drivesswitch 130 into an on state such that current flows through resistors101 and 103, switch 130 and capacitor 102. In one example, the values ofresistors 101 and 103 are selected such that when switch 130 is in an onstate, the time constant of the capacitor 102 and combined resistance ofswitch 130 and resistors 101 and 103 is less than 1 second.

In the example of FIG. 1, high voltage resistors 101 and 103 providesurge protection for the discharge circuit 104 since each resistor has avalue typically in the 100 kOhm to 800 kOhm range. In some embodiments,these resistors are safety certified to allow connection between theinputs terminals 120 and 140 prior to input fuse 105. In one example,resistors 101 and 103 may also provide some protection for the dischargecircuit 104 in the event that the discharge circuit 104 fails. Forinstance, a failure in discharge circuit 104 in may result in a shortcircuit such that the impedance between terminals 125 and 126 issubstantially zero. However, since the resistors 101 and 103 can berated to sustain continuous high voltage conditions, this failure ofdischarge circuit 104 is safe for the power system 100. For this reason,if resistors 101 and 103 are safety certified, discharge circuit 104itself is not required to be safety certified since it is inherentlyprotected by the presence of resistors 101 and 103.

FIG. 2 shows a more detailed block diagram of an example dischargecircuit 204 that in one example could be discharge circuit 104 inFIG. 1. For clarity of the description below, FIG. 2 includes certainexternal elements such as electrical energy source 260, input voltage221, input terminals 220 and 240, resistors 201 and 203, and capacitance202 which in one example could be similar to electrical energy source160, input voltage 121, input terminals 120 and 140, resistors 101 and103, and capacitance 102, respectively, of FIG. 1.

As shown in the depicted example, discharge circuit 204 includes controlcircuit 228 and switch 230. In the example, switch 230 is an ac switchthat includes two n-channel MOSFETs 222 and 223 with their respectivedrains coupled to terminals 225 and 226 and sources coupled together atnode 270 as the discharge circuit 204 internal ground or zero voltreference node. It is appreciated that in other examples the MOSFETs 222and 223 could also be depletion mode MOSFETs configured with a differentdrive circuit in accordance with the teachings of the present invention.It is noted that in the example, the discharge circuit has only twoterminals 225 and 226 coupled to external circuitry. In the example,operating power for the discharge circuit 204 is derived from highvoltage current sources 224 and 229. It is appreciated that in oneexample, high voltage current sources 224 and 229 could be formed frompart of the semiconductor structure of re-channel MOSFETs 222 and 223,(such as for example as illustrated in U.S. Pat. No. 5,285,369)respectively, and would then be regarded as being part of switch 230.However for explanation purposes, they are shown as separate currentsources for the purpose of this description.

As shown in the illustrated example, current sources 224 and 229 arecoupled to internal supply block 227, which generates internal supplyvoltage V_(DD) that is decoupled internally with capacitor 271. It isappreciated that in other examples capacitor 271 could be an externalcapacitor. In an example where capacitor 271 is external to thedischarge circuit 204, it is appreciated that discharge circuit 204would then have at least 4 terminals including terminals 225 and 226,one additional terminal being the node 270 as the ground reference forthe discharge circuit, and a terminal being the V_(DD) supply rail 259.It is appreciated that, in general, the operating power for thedischarge circuit to operate could also be derived in other ways, suchas for example from high voltage current sources separately coupled toexternal nodes 220 and 240 for example, while still benefiting from theteachings of the present invention.

As shown the example depicted FIG. 2, current sources 224 and 229 arecoupled to timer and control block 273. In one example, theseconnections between current sources 224 and 229 and timer and controlblock 273 can be used to detect whether electrical energy source 260 iscoupled to input terminals 220 and 240. In the example, the ac voltagegenerated by electrical energy source 260 will periodically reverse inpolarity. Depending on the polarity of the voltage across inputterminals 220 and 260, one of current sources 224 and 229 will be unableto supply current.

For instance, in one example, at the time that the electrical energysource 260 reverses polarity, the voltage across terminals 225 and 226will be so low such that neither current source 224 nor current source229 will be able to supply current to the internal decoupling capacitor271. In one example however, if the electrical energy source 260 isdisconnected, then the polarity of voltage between terminals 220 and 260will no longer reverse in polarity periodically and one of currentsource 224 or 229 will be able to supply current continuously as long asa voltage is present on the capacitor 202 is sufficiently large for theoperation of the current sources 224 and 229. In one example timer andcontrol circuit block 273 can sense that one of current sources 224 or229 is able to continually supply current for an extended period thatcould in one example be at least 20 milliseconds. The timer and controlcircuit 273 could then determine that the electrical energy source hasbeen disconnected and drive switch 230 into an on state in accordancewith the teachings of the present invention.

In another example, discharge circuit 204 is configured such that theinternal supply rail V_(DD) 259 is coupled to be discharged to a levelbelow an under voltage or reset level V₁ in response to the electricalenergy source voltage reverses in polarity when the electrical energysource is coupled to the input terminals 220 and 240. Under theseconditions, where V_(DD) 259 is reduced to below an under voltage orreset level, in one example this event triggers a reset of the timer incircuit block 273. If the timer in circuit block 273 is not reset for anextended period, such as for example at least 20 milliseconds, in oneembodiment this indicates that an ac source of electrical energy, forexample 260, has been disconnected and in one example control circuit273 could then drive switch 230 into an on state.

It is appreciated that in other examples, switch 230 could be configuredas a current source circuit, such as for example by limiting the currentin MOSFETs 223 and 222 to a specific value either by controlling thegate drive to MOSFETs 223 and 222, or by sizing the MOSFETs 223 and 222to inherently limit the current flowing to a maximum value. It isappreciated that if switch 230 behaves as a current source, resistors201 and 203 for example would not be necessary and the discharge circuit204 itself would regulate the value of the discharge current flowing inswitch 230. In such an example, it could be necessary to have thedischarge circuit qualified as a safety rated circuit.

The waveforms of FIG. 3 illustrate generally operation of an exampledischarge circuit with reference to FIGS. 1 and 2. In particular, FIG. 3shows two example waveforms 388 and 389. In one example, waveform 389 isan example voltage waveform generated by electrical energy source 160 or260 in FIGS. 1 and 2. In the description of FIGS. 3 and 4 below,reference is made with respect to both FIGS. 1 and 2, but it isunderstood that in one example, discharge circuits 104 and 204 could beequivalent and therefore used interchangeably. In FIG. 3, examplewaveform 388 is the voltage across C_(VDD) 271 in FIG. 2. In region 390,the voltage between terminals 225 and 226 is too low for either currentsource 224 or 229 to be able to supply the operating current requirementof discharge circuit 204.

For this description, it is assumed that a positive value of voltagewaveform 389 corresponds to the voltage of terminal 225 being higherthan terminal 226. Therefore when the magnitude of the voltage value ofwaveform 389 is high enough, in region 385, current source 224 iscapable of supplying enough current to allow capacitor C_(VDD) to becharged up to level 382 V₃ at time 391 for example. In one exampleinternal supply block 227 then regulates a voltage across capacitorC_(VDD) 271 to be substantially equal to V₃ 382 as shown by thesubstantially flat portion of waveform 388 in region 385. In oneexample, this regulation is achieved by controlling current sources 224and 229, as represented with signal lines 238 and 239, either bylinearly controlling the current flow or with an on/off or hystereticmode of control. It is recognized that in another example, waveform 388in region 385 would not look flat but could be a sawtooth shape if ahysteretic mode of control of current sources 224 and 229 is used. Inone example V₃ 382 is substantially equal to 5.8 Volts.

In one example, when voltage waveform 389 is negative, current source229 is active in region 386. Internal supply block 227 thereforeregulates the current flowing through current sources 224 and 229 toonly that required to operate the discharge circuit 204. In one examplethe total current required to operate the discharge circuit 204 is lessthan 30 μA. This ensures that the impedance between terminals 225 and226 is high on average during the time that an electrical energy source,such as for example electrical energy source 260, generating waveform389 is connected to the power converter in which the discharge circuit204 is used.

When the magnitude of voltage waveform 389 becomes too low for eithercurrent source 224 or 229 to charge C_(VDD) 271, such as for example inregion 387, in one example V_(DD) 381 falls below a lower under voltageor reset threshold voltage V₁ 383, which in one example is the thresholdused to reset a timer in timer and control circuit 273. In one exampleV₁ 383 is approximately 3 Volts.

FIG. 4 shows example waveforms 491 and 488, which in one example couldoccur when, in FIGS. 1 and 2, electrical energy sources 160 and 260,respectively, are disconnected from input terminals 120, 140 and 220,240 at time 494. For clarity of the explanation below, reference is madeto the discharge circuit 204 in FIG. 2. It is assumed that at timesafter 494, zero current is flowing in fuse 205 and the only path forcurrent to flow to discharge capacitor 202 is therefore throughresistors 201 and 203 and discharge circuit 204. In the example, theoperation prior to time 494 is very similar to that shown in anddescribed with reference to FIG. 3 above.

Continuing with the example illustrated in FIG. 4, at time 494, theelectrical energy source 260 is disconnected. The voltage acrosscapacitor 202, however, remains at a final value 496 of the electricalenergy source 260 just prior to time 494 as illustrated by waveform 491.After an extended period of time 495, which in one example isapproximately 20 milliseconds, the timer in timer and control block 273has not been reset. In one example, transistors 222 and 223 are thendriven into an on state allowing current to flow in resistors 201 and203, transistors 222 and 223, and capacitor 202. The voltage acrosscapacitor 202 therefore drops at a rate determined by the totalresistance of resistors 201 and 203, switch 230 and the capacitance ofcapacitor 202. For explanation purposes, this is illustrated in FIG. 4with an approximately linear reduction of waveform 491 during period497. It is understood, however, that the reduction would actually followan RC discharge characteristic determined by the total resistance andcapacitance of the discharge path.

As shown in the depicted example, the internal supply voltage 488 ofdischarge circuit 204 also decays as shown by waveform 488 during period497, to a lower voltage threshold V₂ 498. In one example V₂ 498 issubstantially equal to 4.8 Volts. In the example, the internal supplyvoltage 488 is then recharged during period 492 back to thresholdvoltage level V₃ 482. In one example this is accomplished by turning offtransistors 222 and 223 in FIG. 2, which allows one of the internalcurrent sources 224 or 229 to recharge internal supply decouplingcapacitor 271. In one example V₃ 482 is substantially equal to 5.8Volts. When the internal supply voltage 488 is charged to voltagethreshold V₃, the transistors 222 and 223 are turned on to continuedischarging the capacitor 202.

In one example, this process of discharging and recharging dischargecircuit supply voltage 488 continues until the voltage remaining onexternal capacitor 202, which is represented by waveform 491, fallsbelow a threshold value 499. At this point, the internal current sources224 and 229 can no longer recharge the internal capacitor 271, even whenMOSFETs 222 and 223 are turned off again at time 493. In this condition,when the internal supply voltage represented by waveform 488 reaches V₂498 at time 493, the voltage 488 continues to reduce at a ratedetermined by the quiescent current consumption of the circuitryinternal to discharge circuit 204. Beyond time point 493, the externalcapacitor 202 is substantially discharged and is therefore notdischarged any further since transistors 222 and 223 are turned off. Inone example, threshold voltage level 499 is in the range of 5 to 10volts.

Therefore, with reference to the circuit of FIG. 2, in one example, thecontrol circuit 228 could alternately be described as driving switch 230with at least two operating modes. A first operating mode whenelectrical energy source 260 is coupled to the input terminals 220 and240. During this first mode, the control circuit 228 drives switch 230such that a high average impedance is present between terminals 225 and226. In one example, the impedance between terminals 225 and 226 is suchthat a current flow between terminals 225 and 226 is less than 30 μAcorresponding to an average impedance of typically greater than 3 MOhmswhen averaged for example over a time period of at least 100 μseconds.

In a second operating mode, the control circuit 228 detects theelectrical energy source 260 has been uncoupled from the input terminals220 and 240. At this point, switch 230 is driven such that a capacitance202 existing between the input terminals 220 and 240 is discharged tobelow a threshold voltage in less than a maximum period of time fromwhen the electrical power or energy source 260 is uncoupled from theinput terminals 220 and 240 in accordance with the teachings of thepresent invention.

FIG. 5 is a schematic illustrating an example of a discharge circuit 504to discharge a capacitance between a power converter input terminalswhen a source of electrical energy is disconnected from the powerconverter input terminals in accordance with the teachings of thepresent invention. In one example, discharge circuit 504 is anintegrated circuit. In one example, the example discharge circuit 504illustrated in FIG. 5 could be the discharge circuits described abovewith reference to FIGS. 1 and 2 generating waveforms similar to thoseshown in FIGS. 3 and 4 in accordance with the teachings of the presentinvention.

As shown in the example depicted in FIG. 5, discharge circuit 504 hasonly two external terminals 525 and 526 coupled to switch 530. In theexample, switch 530 is an ac switch that includes two MOSFETs 522 and523. It is appreciated that in other examples, switch 530 could includeother types of switches such as JFET switches or the like while stillbenefiting from the teachings of the present invention. In one example,regulator circuit 596 generates a supply rail 559 having voltage V_(DD)decoupled with capacitor 571. Control circuit 528 is powered from supplyrail 559 and provides a gate drive output 597 to drive switch 530.Comparators 590, 591 and 592 monitor supply rail voltage 559. If supplyrail 559 is below V3, comparator 590 output drives current sources 524and 529 to supply current to regulator block 596. If supply rail 559 isabove V3, comparator 590 output drives current sources 524 and 529 off.If supply rail 559 is below V2, comparator 591 output provides a highsignal to gate drive logic block 595 to turn off switch 530. If supplyrail 559 is above V2, comparator 591 output provides a low signal togate drive logic block 595.

In one example, line sense block 593 is coupled to timer block 594 toreset timer 594 every time the voltage between terminals 525 and 526falls below a threshold voltage level. In one example, if the voltagebetween terminals 525 and 526 does not fall below a threshold voltagelevel for a threshold period of time, timer output signal 598 is coupledto gate drive logic block 595 to drive switch 530 into an on state. Ifsupply rail voltage 559 falls below voltage threshold V1, PU_resetsignal 599 is coupled to reset timer 594 and all other circuitry withincontrol circuit 528.

In another example, line sense block 593 could be eliminated altogetherand PU_reset signal 599 could instead be coupled to input 589 of timercircuit 594. In that example, the power up reset event itself is used toreset timer circuit 594 such that if supply rail 559 does not fall belowthreshold voltage level V1 for more than a threshold period of time,timer output signal 598 is coupled to gate drive logic block 595 tocommand that switch 530 is turned on.

FIG. 6 shows generally a flowchart 660 illustrating an example methodfor discharging a capacitance between power system input terminals whena source of electrical energy is disconnected from the power systeminput terminals. In one example FIG. 6 could describe the operation ofcircuits 104 and 204 in FIGS. 1 and 2 above. The use of terms V₁, V₂ andV₃ can in one example be assumed equivalent to voltage levels 383/483,498 and 382/482 in FIGS. 3 and 4. Operation starts in block 661 and inblock 662 Q1 and Q2 are in an off state. In one example, Q1 isequivalent to MOSFET 222 and Q2 is equivalent to MOSFET 223 in FIG. 2.In block 663, the timer is reset, which in one example could be thetimer described above with reference to timer and control block 273. Inblock 664, if V_(DD) is less than V₁, then the circuit attempts torecharge C_(VDD), for example C_(VDD) 271 in FIG. 2, and returns toblock 662. If, however, V_(DD) is greater than V₁, then the circuitchecks if V_(DD) is less than V₃ in block 665. If not, in the example,block 666 checks if the internal timer time is up and if so, it isdetermined that for example electrical energy source 260 has beendisconnected and both Q1 and Q2 are turned on in block 667. In block668, it is continually checked whether V_(DD) is greater than V₂ and ifso, the condition of Q1 and Q2 being in an on state persists. However,as soon as V_(DD) is no longer greater than V₂, Q1 and Q2 are turned offin block 669. The operation then returns to block 665, where again it isdetermined whether V_(DD) is less than V₃. If it is, then block 670checks if V_(DD) is less than the lower under voltage or reset voltagethreshold of V₁, in which case then the circuit attempts to rechargeC_(VDD) in block 672 and returns to block 662. Otherwise, C_(VDD) isrecharged in block 671 and operation returns to block 665.

FIG. 7 is an example circuit schematic of a power converter 700 circuitemploying a control circuit 709 and switch 710 coupled to transferenergy from an input to an output 730 of the power converter during afirst operating condition and coupled such that the switch 710 conductsa current without transferring energy from an input to an output of thepower converter under a second operating condition.

In the example, converter 700 is a boost converter. In one example, theboost converter 700 could be used to perform a power factor correctionfunction as will be known to one skilled in the art. Power converter 700is coupled to a source of electrical energy or electrical power 760 andincludes a fuse 705 coupled between EMI capacitor 702 and input terminal740 of power converter 700. In the example, other EMI filter componentblock 706 is coupled to bridge rectifier circuit 707. The output ofbridge rectifier circuit 707 couples to boost converter circuit 711 suchthat, during normal operation, control circuit 709 drives switch 710 toregulate the flow of energy from power converter 700 input terminals 720and 740 to output 730 when the electrical energy source 760 is coupledto the input of the power converter 700.

In one example, control circuit 709 is coupled to detect when theelectrical energy source 760 is disconnected from the input of the powerconverter 700 by sensing, for example, a current in resistor 775. Asshown in the depicted example, resistor 775 is coupled between an outputof rectifier circuit 707 and controller 709. While electrical energysource 760 is coupled to the input of the power converter 700, voltageVdc 731 appearing across the output of rectifier circuit 707 is a fullwave rectified but unsmoothed version of the ac input voltage 721 asrepresented by waveform 732 shown in FIG. 7. When electrical energysource 760 is uncoupled from the input of the power converter 700,however, Vdc 731 will become a steady dc value determined by the valueof the electrical energy source 760 voltage at the moment ofdisconnection. In one example the controller 709 is coupled to detect acurrent flowing in resistor 775 as a way to detect whether Vdc is a fullwave rectified voltage such as represented by waveform 732, or steady dcvoltage level and therefore whether electrical energy source 760 iscoupled or uncoupled to the input of the power converter 700. In oneexample, this could be accomplished by using timing techniques similarto the one used in discharge circuit 204. It is appreciated thatdetection of this type could also be accomplished by coupling controller709 to a node before rectification circuit 707 while still benefitingfrom the teachings of the present invention.

In an alternative description of power converter 700, control circuit709 can be described as driving switch 710 with at least two operatingmodes. A first operating mode when electrical energy source 760 iscoupled to the input of the power converter 700. During this first mode,the control circuit 709 drives switch 710 to regulate a flow of energyfrom the input to the output of power converter 700. The first modeincludes a condition where the energy flow is regulated to substantiallyzero during a standby or shutdown mode of the control circuit 709. Inthe standby or shutdown condition, the control circuit 709 drives switch710 into an off state.

In a second operating mode, the control circuit 709 detects that theelectrical energy source 760 has been uncoupled from the input to thepower converter 700. Switch 710 is then driven such that current flowsin the switch 710. A capacitance 702 existing between the inputterminals 720 and 740 of the power converter 700 is then discharged tobelow a threshold voltage in less than a maximum period of time fromwhen the electrical power or energy source 760 is uncoupled from theinput terminals 720 and 740 of the power converter 700 in accordancewith the teachings of the present invention. The maximum period of timeis unaffected by the amount of energy flow between power converter 700input and output immediately prior to the electrical energy source beinguncoupled from the input terminals of the power converter.

It is noted that discharging the capacitance 702 could in one example beaccomplished by driving switch 710 on continuously until the requiredlevel of discharge of capacitance 702 is achieved. In another example,discharging the capacitance 702 could be accomplished by driving theswitch 710 on and off until the required discharge of capacitance 702 isachieved such that energy is still transferred from an input to anoutput of the power converter 700 during the discharging period inaccordance with teachings of the present invention. In one example, theflow of energy from input to output of the power converter 700 duringthis discharge period may be regulated or unregulated by control circuit709.

FIG. 8 is another example circuit schematic of a power converter 800circuit employing a control circuit 813 in accordance with the teachingsof the present invention. As shown in the depicted example, controlcircuit 813 is coupled to switches 815, 816 and 817. In the example,switches 816 and 815 are the two switches of a 2 switch forwardconverter forming the main power supply, which may be utilized, forexample, in a personal computer power converter. In the example, switch817 is the switch of a flyback converter, which for example could formpart of a standby power supply in a personal computer power converter.Both main and standby power conversion stages can be referred to asdc-dc converters since the output voltage of power conversion stage 811is a substantially constant dc value. In one example, switch 817 isdriven by control circuit 813 to transfer energy from an input of powerconverter 800 to an output 819 of the power converter during a firstoperating condition and coupled such that switch 817 conducts a currentwithout transferring energy from the input of converter 800 to theoutput 819 under a second operating condition.

As shown in the depicted example, power converter 800 is coupled to asource of electrical energy or electrical power 860 and includes a fuse805 coupled between EMI capacitor 802 and input terminal 840 of powerconverter 800. In the example, other EMI filter components block 806 iscoupled to bridge rectifier circuit 807. As shown in the example, theoutput of bridge rectifier circuit 807 is coupled to boost convertercircuit 811. The output of boost converter circuit 811 is coupled toforward and flyback converters 812.

During one mode of operation, control circuit 813 drives switches 815and 816 to regulate the flow of energy from power converter 800 inputterminals 820 and 840 to output 818 when the electrical energy source860 is coupled to the input of the power converter 800. In one examplecontrol circuit 813 is coupled to detect when the electrical energysource 860 is disconnected from the input of the power converter 800 bysensing a current, for example, in resistor 875. As shown in thedepicted example, resistor 875 is coupled between an output of rectifiercircuit 807 and controller 813. While electrical energy source 860 iscoupled to the input of the power converter 800, voltage Vdc 831appearing across the output of rectifier circuit 807 is a full waverectified but unsmoothed version of the ac input voltage 821 asrepresented by waveform 832. When electrical energy source 860 isuncoupled from the input of the power converter 800 however, Vdc 831will become a steady dc value determined by the value of the electricalenergy source 860 voltage at the moment of disconnection. In oneexample, the controller 813 is coupled to detect a current flowing inresistor 875 as a way to detect whether Vdc 831 is a full wave rectifiedor steady dc voltage level and therefore whether electrical energysource 860 is coupled or uncoupled to the input of the power converter800. In one example, this could be accomplished by using timingtechniques similar to the one used in discharge circuit 204. It isappreciated that detection of this type could also be accomplished bycoupling controller 813 to a node before rectification circuit 807 whilestill benefiting from the teachings of the present invention.

In an alternative description of power converter 800, control circuit813 can be described as driving switches 815 and 816 with at least twooperating modes. A first mode when electrical energy source 860 iscoupled to the input of the power converter 800. During this first mode,the control circuit 813 drives switches 815 and 816 to regulate a flowof energy from the input to the output 818 of power converter 800. Thefirst mode includes a condition where the energy flow is regulated tosubstantially zero during a standby or shutdown mode of the controlcircuit 813. In the standby or shutdown condition, the control circuit813 may drive switches 815 and 816 into an off state.

In a second operating mode, the control circuit 813 detects that theelectrical energy source 860 has been uncoupled from the input to thepower converter 800. Switches 815 and 816 are then driven such that acapacitance 802 existing between the input terminals of the powerconverter is discharged to below a threshold voltage in less than amaximum period of time from when the electrical power or energy source860 is uncoupled from the input terminals 820 and 840 of the powerconverter 800 in accordance with the teachings of the present invention.The maximum period of time is unaffected by the amount of energy flowbetween power converter 800 input and output immediately prior to theelectrical energy source 860 being uncoupled from the input terminals ofthe power converter 800.

It is noted that discharging the capacitance 802 could in one example beaccomplished by driving the switches 815 and 816 on and off until therequired discharge of capacitance 802 is achieved such that energy isstill transferred from an input to an output of the power converterduring the discharging period in accordance with the teachings of thepresent invention. In one example, the flow of energy from input tooutput of the power converter during this discharge period may beregulated or unregulated by control circuit 813.

In an alternative example using power converter 800, control circuit 813can also be described as driving switch 817 with at least two operatingmodes. A first mode when electrical energy source 860 is coupled to theinput of the power converter 800. During this first mode, the controlcircuit 813 drives switch 817 to regulate a flow of energy from theinput to the output 819 of power converter 800. The first mode includesa condition where the energy flow is regulated to substantially zeroduring a standby or shutdown mode of the control circuit 813. In thestandby or shutdown mode, the control circuit 813 drives switch 817 inan off state.

In a second operating mode, the control circuit 813 detects that theelectrical energy source 860 has been uncoupled from the input to thepower converter 800. Switch 817 is then driven such that the capacitance802 existing across the input terminals of the power converter 800 isdischarged to below the threshold voltage in less than a maximum periodof time from when the electrical power or energy source 860 is uncoupledfrom the input terminals 820 and 840 of the power converter 800 inaccordance with the teachings of the present invention.

It is noted that discharging the capacitance 802 could in one example beaccomplished by switching the switch 817 on continuously until thedischarge of capacitance 802 is achieved. In another example,discharging the capacitance 802 could be accomplished by switching theswitch 817 on and off until the required discharge of capacitance 802 isachieved such that energy is still transferred from an input to anoutput of the power converter during the discharging period inaccordance with the teachings of the present invention. In one example,the flow of energy from input to output of the power converter duringthis discharge period may be regulated or unregulated by control circuit813.

It is noted that the examples of FIGS. 7 and 8 discussed fuses 705 and805 are positioned between the input terminals of power converters 700and 800 and capacitors 702 and 802, respectively. In this arrangement,there is discharge path between capacitance 702 and switch 710 in FIG.7, and between capacitance 802 and switches 816/815 or 817 in FIG. 8,even if the respective input fuses are in an open circuit condition. Itis appreciated that with this arrangement of the input fuse 705 or 805,a further example of a circuit, not shown, benefiting from the teachingsof the present invention could be a switch coupled directly across theoutput terminals of rectification circuits 707 or 807 in FIG. 7 or 8,respectively. Such a switch could perform a similar discharge functionto that performed by switches 710, 815, 816 and 817 described above andcould be driven from controllers 709 or 813. In one example this switchcould be coupled to a current limiting resistor to limit maximum peakcurrent in the switch during a discharge event.

It is appreciated that other examples of circuits benefiting from theteachings of the present invention could include, for example, usingcircuitry internal to control circuit 709 in FIG. 7 and coupled toresistor 775 as a current path for discharging a capacitance 702 thatexists between the input terminals of power converter 700 whenelectrical energy source 760 is uncoupled from the input of powerconverter 700. In one example, this could be accomplished by usingtechniques similar to the discharge circuit 204 but with a single dcswitch and a single current source in place of the ac switch formed byswitches 222 and 223 and current sources 224 and 229. In one example,the current flowing in resistor 775 while electrical energy source 760is coupled to the input of power converter 700 could also provide astarting current for the operation of the control circuit 709. In theexample, the start up current is supplied only during a start up phaseof control circuit 709 after which a supply winding, not shown, on theboost inductor or other magnetic components would take over providingsupply current to control circuit 709.

Yet another example could include, for example, using circuitry internalto control circuit 813 in FIG. 8 and coupled to resistor 875 as acurrent path for discharging a capacitance 802 that exists between theinput terminals of power converter 800 when electrical energy source 860is uncoupled from the input of power converter 800. In one example, thiscould be accomplished by using techniques similar to the dischargecircuit 204 but with a single dc switch and a single current source inplace of the ac switch formed by switches 222 and 223 and currentsources 224 and 229. In one example, the current flowing in resistor 875while electrical energy source 860 is coupled to the input of powerconverter 800 could provide a starting current for the operation of thecontrol circuit 813. In the example, the start up current function isactive only during a start up phase of control circuit 813 after which asupply winding, not shown, within the power converter 800 on the boostinductor or other magnetic components for example, would take overproviding supply current to control circuit 813 when the voltage on thatwinding reaches a threshold value.

It is also appreciated that power converters 700 and 800 could haveother modes of operation other than the modes described above forexplanation purposes, including, for example, specific protection modeswhen a fault occurs, while still benefitting from the teachings of thepresent invention.

FIG. 9 shows generally a flowchart 960 illustrating an example methodfor a control circuit to discharge a capacitance between input terminalsof a power converter when a source of electrical energy is disconnectedfrom the power converter input terminals. In one example, the methodillustrated in FIG. 9 is similar to those described above with respectto FIGS. 7 and 8. As shown, the power converter starts in block 961. Inblock 962, it is determined whether a source of electrical energy iscoupled to the power converter input. If it is, the control circuitdrives a switch to regulate an energy flow from an input to an output ofthe power converter. It is appreciated that in one example, the energyflow could be regulated to substantially zero. The output of block 963is then connected to the input of decision block 962. If the electricalenergy source is not connected to the power converter input, then theoutput of block 962 is connected to block 964, in which the controlcircuit drives a switch to discharge a capacitance connected betweenpower converter input terminals to a threshold level within a maximumperiod of time in accordance with the teachings of the presentinvention.

FIG. 10 shows another example circuit benefiting from the teachings ofthe present invention. It is appreciated that the example circuitdiagram shown in FIG. 10 shares similarity with the example circuitdiagram shown in FIG. 1. However, discharge circuit 1004 in FIG. 10 hasbeen integrated within Main, Standby & Discharge control circuit 1013.As shown in the depicted example, control circuit 1013 forms part ofintegrated circuit 1014, which also includes switches 1015, 1016 and1017. Functionally, the operation of discharge circuit 1004 is, in oneexample, very similar to the discharge circuits 104 and 204 describedabove with respect to FIGS. 1 and 2, respectively. In an examplepractical realization of power converter 1000, the integration ofdischarge circuit 1004 in this way can offer cost and printed circuitboard area savings. It is appreciated that in other examples, dischargecircuit 1004 could be integrated with PFC controller 1009. In general,discharge circuit 1004 could be integrated with any controller circuitsuch as an LLC converter, full and half bridge converters, SEPICconverters and CUK converters to name a few.

FIG. 11 shows one example of a discrete component realization of adischarge circuit 1104 in accordance with the teachings of the presentinvention. As shown, capacitor 1157 and diodes 1156 and 1158 form acharge pump or capacitive dropper power supply that generates a voltageacross capacitor 1160 when ac electrical energy source 1160 is coupledto input terminals 1120 and 1140. Current therefore flowing throughresistor 1155 and into base of transistor 1154, holds transistor 1154 inan on state while electrical energy source 1160 remains coupled to inputterminals 1120 and 1140. The gates of MOSFETs 1122 and 1123 are coupledto the collector of transistor 1154 through diodes 1152 and 1153. Thisensures that the gate voltage of MOSFETs 1122 and 1123 relative tocircuit ground 1150 is below the gate threshold voltage of the MOSFETsand that the MOSFETs 1122 and 1123 are in an off state when transistor1154 is in an on state. If, however, ac electrical energy source 1160 isuncoupled from input terminals 1120 and 1140, the charge pump circuit nolonger provides energy to capacitor 1160 and capacitor 1160 dischargesat rate set by the RC time constant of resistors 1161 and 1155, andcapacitor 1160.

After an extended period of time, which in one example could beapproximately 20 milliseconds, selected through the choice of componentvalues for capacitor 1160, and resistors 1155 and 1161, transistor 1154turns off. At this time, depending on the polarity of the voltage acrosscapacitor 1102, the gate voltage of either MOSFET 1122 or MOSFET 1123rises to the gate voltage threshold and is eventually clamped by eitherzener 1178 or zener 1179.

In an example where the voltage on input terminal 1120 is higher thanthe voltage on input terminal 1140, the gate of 1122 will be pulled highturning MOSFET 1122 on. Current will then flow from capacitor 1102,through resistor 1101, through the channel of MOSFET 1122, through thebody diode of MOSFET 1123 (which is an inherent part of thesemiconductor structure of MOSFET 1123 as will be known to one skilledin the art), through resistor 1103 and then back to capacitor 1102, thusforming a discharge current path in accordance with the teachings of thepresent invention. When the voltage across capacitor 1102 reaches alower threshold value, below which the gate voltage of 1122 falls belowthe gate threshold voltage of 1122, the MOSFET 1122 turns off and aresidual voltage typically in the range of 5 to 10 volts depending onthe type of MOSFET used for 1122, will remain on capacitor 1102.

It is noted that the descriptions above have been focused on powerconverters for explanation purposes where input and output energy isprimarily electrical. FIG. 12 shows an example of a motor control powerconverter or power system where input power and energy is electrical,but is converted to primarily mechanical energy and power at the output.In the illustrated example, it is appreciated that the discharge circuit1204 shares many similarities in feature and functions with controlcircuits 104 and 204 described above. Although the example depicted inFIG. 12 illustrates the use of a boost converter 1211, it is appreciatedthat in other examples, the inclusion of a power conversion stage 1211would not necessarily be required in accordance with the teachings ofthe present invention. It is appreciated that in other examples,discharge circuit 1204 could be integrated within motor controller 1214or even the power switch circuit 1217 that is coupled to motor windings1219. It is appreciated that in other examples, the power system couldbe one where the input power and energy is electrical but is convertedat the output of the power system to primarily optical or light outputenergy, such as for example in an LED driver power system.

FIG. 13 shows a schematic of another example of a discharge circuit todischarge a capacitance between input terminals of a power converterwhen a source of electrical energy is disconnected from the powerconverter input terminals in accordance with the teachings of thepresent invention. Electrical energy source 1360, which in the exampleis an ac voltage source, is coupled to input terminals 1320 and 1340. Asillustrated in the depicted example, while ac voltage source 1360 iscoupled to input terminals 1320 and 1340, current flows in capacitor1303, through rectifier bridge 1305 and provides a current in a winding1331 of relay 1330. In the example, relay 1330 is a normally closed typewhere the relay switch 1332 is open when current flows in winding 1330.When energy source 1360 is uncoupled from terminals 1320 and 1340,current ceases to flow in capacitor 1303 and rectification bridge 1305.The current flowing in winding 1331 then falls to substantially zero ina time determined by the value of capacitor 1301. When the current inwinding 1331 falls below a threshold level, switch 1332 then closes anddischarges capacitor 1302. In one example, current limiting resistorscould be coupled in series with switch 1332 to limit peak currents inswitch 1332 when the electrical energy source is first coupled to theinput terminals and when the switch is closed to discharge capacitor1302.

In general is it appreciated that all the teachings of the presentinvention discussed above with respect to FIGS. 1 to 13 can be appliedto any system coupled to a source of electrical energy where thecapacitance coupled between input terminals of the circuit poses a riskof electrical shock if left charged when the source of electrical energyis uncoupled from the input to the system.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific voltages,currents, frequencies, power range values, times, etc., are provided forexplanation purposes and that other values may also be employed in otherembodiments and examples in accordance with the teachings of the presentinvention.

These modifications can be made to examples of the invention in light ofthe above detailed description. The terms used in the following claimsshould not be construed to limit the invention to the specificembodiments disclosed in the specification and the claims. Rather, thescope is to be determined entirely by the following claims, which are tobe construed in accordance with established doctrines of claiminterpretation. The present specification and figures are accordingly tobe regarded as illustrative rather than restrictive.

What is claimed is:
 1. A power converter, comprising: an energy transferelement coupled between an input of the power converter and an output ofthe power converter; and a circuit coupled to the input of the powerconverter, wherein the circuit further comprises: a control circuitcapable of detecting whether an AC voltage source is coupled to theinput of the power converter; and a switch coupled to the controlcircuit, wherein the control circuit is capable of detecting whether theAC voltage source is uncoupled from the input of the power converterwithin a first predetermined maximum period of time, wherein the controlcircuit is coupled to drive the switch in a first operating mode whenthe AC voltage source is coupled to the input of the power converter,wherein the control circuit is coupled to drive the switch in a secondoperating mode when the control circuit detects the AC voltage source isuncoupled from the input of the power converter and is capable ofdischarging a capacitance coupled between input terminals of the powerconverter to a threshold voltage level through a discharge path andthrough the switch within a second predetermined maximum period of time,and wherein an RC time constant of the discharge path is less than orequal to one second and the threshold voltage level is less than orequal to ten volts.
 2. The power converter of claim 1, wherein theswitch is coupled to provide a starting current for operation of thecircuit during a start up phase of the circuit.
 3. The power converterof claim 2, wherein the starting current flows through the switch. 4.The power converter of claim 2, wherein the switch comprises a firsttransistor coupled to a second transistor, and one of the first andsecond transistors is turned on to provide the starting current.
 5. Thepower converter of claim 4, wherein one of the first and secondtransistors is turned on to discharge the capacitance.
 6. The powerconverter of claim 1, wherein the capacitance coupled between the inputterminals of the input of the power converter comprises one or more Xcapacitors.
 7. The power converter of claim 1, wherein the controlcircuit and the switch are included in an integrated circuit.
 8. A powerconverter, comprising: an energy transfer element coupled between aninput of the power converter and an output of the power converter; and acircuit coupled to the input of the power converter, wherein the circuitfurther comprises: a control circuit capable of detecting whether an ACvoltage source is coupled to the input of the power converter; and abidirectional switch comprising a first transistor coupled to a secondtransistor, the bidirectional switch further coupled to the controlcircuit, wherein the control circuit is capable of detecting whether theAC voltage source is uncoupled from the input of the power converterwithin a first predetermined maximum period of time, wherein the controlcircuit is coupled to drive the bidirectional switch in a firstoperating mode when the AC voltage source is coupled to the input of thepower converter, wherein the control circuit is coupled to drive thebidirectional switch in a second operating mode when the control circuitdetects the AC voltage source is uncoupled from the input of the powerconverter, and is capable of discharging a capacitance coupled betweeninput terminals of the power converter to a threshold voltage levelthrough a discharge path and through the bidirectional switch within asecond predetermined maximum period of time, and wherein an RC timeconstant of the discharge path is less than or equal to one second andthe threshold voltage level is less than or equal to ten volts.
 9. Thepower converter of claim 8, wherein the bidirectional switch is coupledto provide a starting current for operation of the circuit during astart up phase of the circuit.
 10. The power converter of claim 9,wherein one of the first and second transistors of the bidirectionalswitch is turned on to provide the starting current.
 11. The powerconverter of claim 10, wherein the starting current flows through one ofthe first and second transistors of the bidirectional switch.
 12. Thepower converter of claim 8, wherein one of the first and secondtransistors of the bidirectional switch is turned on to discharge thecapacitance.
 13. The power converter of claim 8, wherein the capacitancecoupled between the input terminals of the input of the power convertercomprises one or more X capacitors.
 14. The power converter of claim 8,wherein the control circuit and the bidirectional switch are included inan integrated circuit.
 15. A power converter, comprising: an energytransfer element coupled between an input of the power converter and anoutput of the power converter; and a circuit coupled to the input of thepower converter, wherein the circuit further comprises: a controlcircuit capable of detecting whether an AC voltage source is coupled tothe control circuit by determining whether a voltage of the AC voltagesource has reversed in polarity; and a switch coupled to the controlcircuit, wherein the control circuit is capable of detecting whether theAC voltage source is uncoupled from the input of the power converterwithin a first predetermined maximum period of time, wherein the controlcircuit is coupled to drive the switch in a first operating mode whenthe AC voltage source is coupled to the control circuit, wherein thecontrol circuit is coupled to drive the switch in a second operatingmode when the control circuit detects the AC voltage source is uncoupledfrom the control circuit to discharge a capacitance coupled betweeninput terminals of the power converter to a threshold voltage levelthrough a discharge path and through the switch within a secondpredetermined maximum period of time, and wherein an RC time constant ofthe discharge path is less than or equal to one second and the thresholdvoltage level is less than or equal to ten volts.
 16. The powerconverter of claim 15, wherein the switch is coupled to provide astarting current for operation of the circuit during a start up phase ofthe circuit.
 17. The power converter of claim 16, wherein the startingcurrent flows through the switch.
 18. The power converter of claim 15,wherein the capacitance coupled between the input terminals of the inputof the power converter comprises one or more X capacitors.
 19. The powerconverter of claim 15, wherein the control circuit and the switch areincluded in an integrated circuit.
 20. The power converter of claim 15,wherein the control circuit detects whether the AC voltage source iscoupled to the control circuit by determining whether the voltage of theAC voltage source has reversed in polarity within a third predeterminedperiod of time.